The aminopolyphosphonates, polycarboxylates or aminopolycarboxylates, such as ethylenediaminetetraacetic acid (EDTA), often used as complexing agents for example in detergents and cleaners are biodegradable only to a small degree.
A cost-effective alternative is the glycine-N,N-diacetic acid derivatives, such as methylglycine-N,N-diacetic acid (MGDA) and salts thereof—e.g. the trialkali metal salts—which have advantageous toxicological properties and are readily biodegradable. The use of MGDA and of related glycine-N,N-diacetic acid derivatives in cleaners, and the syntheses thereof are described e.g. in WO-A 94/029421 or U.S. Pat. No. 5,849,950. For a cost-effective production of the glycine-N,N-diacetic acid derivatives, high requirements are placed on the yield of the individual synthesis steps and purity of the isolated intermediate products.
MGDA is prepared in particular by reacting iminodiacetonitrile with acetaldehyde and hydrocyanic acid or of alpha-alanine nitrile with formaldehyde and hydrocyanic acid and alkaline hydrolysis of the methylglycinediacetonitrile (MGDN) obtained as intermediate product with sodium hydroxide solution, giving the trisodium salt of MGDA. In order to achieve high MGDA yields and purities, MGDN is generally isolated as an intermediate product and used as pure substance in the subsequent hydrolysis step.
A problem with the hydrolysis of alkylglycinenitrile-N,N-diacetonitriles is their thermal lability, especially in an alkaline medium. As a result of the sterically demanding alkyl substitution, back-cleavage reactions are favored. Consequently, processes have been developed which as far as possible lead to low by-product forms of MGDA and its salts.
An improved process for the preparation of low by-product salts of MGDA is described in WO 2006/120129. The more modern production processes generally lead to about 35-40% strength by weight aqueous solutions, from which the salts are then prepared in flowable form.
One of the known work-up processes in the prior art is the conversion of such aqueous solutions in a spray-tower. This produces predominantly amorphous powders with a residual moisture in the order of magnitude of for example 5% by weight. Although higher residual moistures are conceivable, they are rather difficult to generate in a spray-tower and are, moreover, also undesired because then upon subsequent storage by the consumer or during processing, clumping of the powders can arise. It is also known that granules do not have such disadvantages and can therefore be processed without problems. However, granule production requires an additional reworking step following powder production in the spray-tower and is therefore relatively expensive. In this reworking step, additional moisture is fed to the powder from the spray-tower, and granulation is carried out with heating and kneading at a residence time in the order of magnitude of one hour via a crystallization. Such a process is described for example in EP-A 08 45 456.